Conventionally, mold flash on the lead frame of an IC package is removed and cleaned by high pressure water jet sputtering or wet etching in chemical solutions.
In the case of water jet sputtering, high pressure water (with several thousand atmospheric pressure) is formed to be a water jet by a fine nozzle. This water jet is then directed to the region (on lead frame of IC package) where mold remnants exist. The water remaining on the IC chips is dried away in the subsequent step.
Another method of removing mold flash is chemical etching. The chips are put in a chemical solution for a certain period of time while the flash part is etched away. The chips are then rinsed and dried in subsequent processes.
The two methods mentioned above, especially the water jet sputtering, are effective and currently used in industry to remove the mold flash from IC packages and other electronic components. However, they have some disadvantages which significantly affect the productivity of flash removal and reliability of the IC chips in manufacturing.
For water jet sputtering, high cost and subsequent drying are the main drawbacks of the technology. In order to obtain high pressure water, a high-price water compressor has to be used to pump water to several thousand atmospheric pressures. Meanwhile, the jet nozzle is worn out in a short period of time (within a few hours) due to the small nozzle size and high water pressure. Use of a high pressure water pump and frequent changing of the jet nozzle increase the cost of the process. Furthermore, the wet chips after flash removal have to be dried in a subsequent process which also increases the cost and reduces the productivity.
The method of wet etching in chemical solutions to remove mold flash also has several problems. First of all, the chemical solution to etch the mold flash away can also be corrosive to the IC package and to the metal lead frame. The chemical solution may penetrate into the IC package to degrade the reliability of the IC chips. Secondly, metal is chemically more active than the mold material. Thereafter, it is difficult to find a chemical solution which is chemically more active to the mold material but less active to the lead metal. This method also requires water rinse and a subsequent drying process. Therefore, this method is even worse than the previous one in terms of cost and productivity.
This invention is aimed to solve the above-mentioned problems by using a laser as an energy source to remove mold flash. It is a dry process in air which does not need sophisticated equipment and a subsequent drying process. Therefore, the new method has high productivity and low cost. Furthermore, this new method does not have pollution such as noise and toxic chemical gases or solutions.
A known laser trimming system for semiconductor integrated circuit chip packages is disclosed in U.S. Pat. No. 5,099,101 to Millerick et al. The device of Millerick et al uses a YAG laser which has a wavelength of 1.06 micrometer. The mold flash is removed by "vaporization " which is a pure thermal process. A disadvantage of this system is that the high temperature rise can cause the mold flash to be carbonized and the lead frame to be oxidized. The existence of the carbon residual can cause a reliability problem because the carbon residual is electrically conductive when the humidity is high. At the same time, the oxidation of the lead frame can cause problems in electrical connection. Because of these disadvantages, this technique is not widely used in industry.
The Millerick et al device also discloses a monitoring system including a CCD camera and an image processor. This system requires illumination and the resolution is limited by the pixel of the image processor. Therefore, the monitoring system suffers from the disadvantage that the pattern edges cannot be defined efficiently with high resolution.